1. Field of the Invention
The present invention relates to an X-ray imaging device using a flat panel detector (FPD), a method, and a program for detecting deviation between the FPD and an X-ray field.
2. Description Related to the Prior Art
Digital radiography using a flat panel detector (FPD) for X-ray imaging has come into widespread use in a medical field in recent years. The FPD has small size, light weight, and a large imaging surface. In an X-ray imaging device having the FPD, an X-ray radiation beam produced by an X-ray generator is applied to patient's body part to be viewed. The X-ray radiation beam passes through the body part of the patient, and the FPD captures the X-ray radiation beam behind the body part. The FPD produces a digital-format radiographic image from the captured X-ray radiation beam. The FPD can immediately apply image processing or enhancement to the radiographic image because the radiographic image is in the digital format. The radiographic image is directly displayed on a monitor. The FPD allows immediate display of the radiographic image on the monitor, without development of an exposed film in a darkroom or read of an imaging plate (IP) with a laser scanner, as conventionally.
The X-ray generator is provided with a collimator unit. The collimator unit has openable and closable collimator leaves that make an X-ray field of the X-ray radiation beam into a rectangular shape, similarly to the imaging surface of the FPD, before the application to patient's body. The collimator leaves cut redundant X-rays from the X-ray radiation beam, and reduce unnecessary X-ray radiation absorption by the patient's body.
It is desirable that the center of the X-ray field formed by the collimator leaves coincide with the center of the imaging surface of the FPD. However, the position of the FPD relative to the X-ray generator is visually checked and adjusted by a radiological technologist, and adjustment accuracy depends on skill of the technologist. Accordingly, it is proposed to capture a preliminary radiographic image for the position adjustment before capturing a diagnostic radiographic image, for the purpose of quantitatively detecting deviation between the imaging surface of the FPD and the X-ray field.
Taking a case of an X-ray imaging device according to Japanese Patent Laid-Open Publication No. 2006-122488 as an example, an image of the rectangular X-ray field is captured. Then, X-directional midpoint coordinates and Y-directional midpoint coordinates of the X-ray field are obtained from the image, to detect the center of the X-ray field.
In this method, however, the whole X-ray field has to be seen in the image to obtain the X-directional midpoint coordinates and the Y-directional midpoint coordinates. Thus, if a part of the X-ray field lies off the imaging surface due to large deviation, the center of the X-ray field is undetectable.
Especially, a portable FPD called as an electronic cassette is disposed between a bed and the patient lying thereon to capture a radiographic image. The FPD is repositioned whenever capturing the image, in accordance with a body size of the patient or the body part to be viewed. The reposition of the FPD tends to cause deviation of the imaging surface of the FPD from the X-ray field.